ServletContext对象详解

ServletContext对象

　　WEB容器在启动时，它会为每个WEB应用程序都创建一个对应的ServletContext对象，它代表当前web应用。
　　ServletConfig对象中维护了ServletContext对象的引用，开发人员在编写servlet时，可以通过ServletConfig.getServletContext方法获得ServletContext对象。
　　由于一个WEB应用中的所有Servlet共享同一个ServletContext对象，因此Servlet对象之间可以通过ServletContext对象来实现通讯。ServletContext对象通常也被称之为context域对象。

多个Servlet通过ServletContext对象实现数据共享

　　范例：Demo2和Demo3通过ServletContext对象实现数据共享

Demo2.java:

package Java.Servlet;

import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;

@WebServlet("/demo2")
public class Demo2 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        resp.setCharacterEncoding("utf-8");
        //1.设置response的编码为utf-8
        resp.setContentType("text/html;charset=UTF-8");
        //2.通知浏览器，以UTF-8的编码打开
        String data = "多个Servlet通过ServletContext对象实现数据共享";
        /**
         * ServletConfig对象中维护了ServletContext对象的引用，开发人员在编写servlet时，
         * 可以通过ServletConfig.getServletContext方法获得ServletContext对象。
         */
        ServletContext context = this.getServletConfig().getServletContext();
        //获得ServletContext对象
        context.setAttribute("data", data);
        //将data存储到ServletContext对象中
    }
}

demo3.java:

package Java.Servlet;

import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;

@WebServlet("/demo3")
public class Demo3 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        resp.setCharacterEncoding("utf-8");
        //1.设置response的编码为utf-8
        resp.setContentType("text/html;charset=UTF-8");
        //2.通知浏览器，以UTF-8的编码打开
        ServletContext context = this.getServletContext();
        String data = (String)context.getAttribute("data");
        //从ServletContext对象中取出数据

        resp.getWriter().println("<h1>data:" + data + "</h1");
    }
}

先运行Demo2，将数据data存储到ServletContext对象中，然后运行Demo3就可以从ServletContext对象中取出数据了，这样就实现了数据共享，如下图所示：

获取WEB应用的初始化参数

　　在web.xml文件中使用<context-param>标签配置WEB应用的初始化参数，如下所示：

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_4_0.xsd"
         version="4.0">
    <context-param>
        <param-name>url</param-name>
        <param-value>jdbc:mysql://localhost:3306/test</param-value>
    </context-param>
</web-app>

获取Web应用的初始化参数，代码如下：

package Java.Servlet;

import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;

@WebServlet("/demo4")
public class Demo4 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ServletContext context = this.getServletContext();

        String url = context.getInitParameter("url");
        resp.getWriter().println("<h1>url:" + url + "</h1>");
    }
}

运行结果如下:

用servletContext实现请求转发

Demo5.class:

package Java.Servlet;

import javax.servlet.RequestDispatcher;
import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;

@WebServlet("/demo5")
public class Demo5 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        String demo5 = "In Demo5";
        resp.getWriter().println(demo5);

        ServletContext context = this.getServletContext();
        RequestDispatcher rd = context.getRequestDispatcher("/demo6");
        rd.forward(req, resp);
    }
}

Demo6.class:

package Java.Servlet;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;

@WebServlet("/demo6")
public class Demo6 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        String demo6 = "In Demo6";
        resp.getWriter().println(demo6);
    }
}

运行结果如下:

(忘记加粗了,尴尬)

利用ServletContext对象读取资源文件

见下篇